The invention relates to the preparation of UV-curable urethane compositions. More particularly, the invention relates to preparing UV-curable urethane compositions from hydroxy-functional resins comprising recurring units of propoxylated allylic alcohols.
Urethane acrylates are known. They can be divided into two categories: monomeric and polymeric. Monomeric urethane acrylates can be made by the reaction of multi-functional isocyanates with hydroxyalkyl acrylates or methacrylates. Polymeric urethane acrylates are made by the reaction of NCO prepolymers with hydroxyalkyl acrylates or methacrylates. The commonly used NCO prepolymers are made from polyether polyols or polyester polyols. See, e.g., U.S. Pat. No. 4,324,575.
Despite their superior resistance to weathering, acrylic polyols are rarely used for making NCO prepolymers. Unlike polyether or polyester polyols, acrylic polyols are made by radical chain polymerization of hydroxyalkyl acrylates or methacrylates and alkyl acrylates or methacrylates. The hydroxyl groups are randomly pendant along the polymer chain, rather than located only at the chain ends. Also, acrylic polyols usually have more than two OH groups per chain. Thus, the reaction of acrylic polyols with diisocyanates often results in gel even under well-controlled conditions.
Isophorone diisocyanate (IPDI) is also known. It offers coatings excellent weathering stability. More importantly, IPDI has two different NCO groups, a primary aliphatic NCO group and a secondary cycloaliphatic NCO group. Under Lewis acid catalysis, the secondary NCO group is considerably more reactive than the primary NCO group. This reactivity difference makes IPDI an ideal candidate for preparing NCO prepolymers because the secondary NCO group reacts first with the OH group leaving the primary NCO group unreacted and available for further reaction. See R. Lomoelder et al., Paintindia, pp. 31-38, November 1998.
Nevertheless, the reaction of IPDI with acrylic polyols often results in gel. New methods for preparing UV-curable acrylic urethanes are needed. Ideally, the method would allow the direct reaction of acrylic polyols with IPDI without gel formation.
The invention is a method of preparing UV-curable urethane compositions. The method comprises two steps. First, isophorone diisocyanate (IPDI) reacts with a hydroxy-functional resin to form an NCO prepolymer. Second, the NCO prepolymer reacts with a hydroxy-functional ethylenic monomer to form the UV-curable urethane composition. The hydroxy-functional resin comprises at least about 2 wt % of recurring units of a propoxylated allylic alcohol. The method of the invention allows the preparation of UV-curable urethane compositions from acrylic polyols without gel.
The method of the invention comprises two steps. First, isophorone diisocyanate (IPDI) reacts with a hydroxy-functional resin to form an NCO prepolymer. Second, the NCO prepolymer reacts with a hydroxy-functional ethylenic monomer.
The hydroxy-functional resin comprises at least about 2 wt % of recurring units of a propoxylated allylic alcohol. Preferably, the hydroxy-functional resin comprises at least about 5 wt % of recurring units of a propoxylated allylic alcohol. More preferably, the hydroxy-functional resin comprises at least about 10 wt % of recurring units of a propoxylated allylic alcohol.
The propoxylated allylic alcohol preferably has the general structure: 
R is hydrogen, a C1-C10 alkyl, or a C6-C12 aryl group; A is an oxypropylene group; and n is an average number of oxyalkylene groups. Preferably, n is from about 1 to about 5. More preferably, n is from about 1 to about 2. Most preferably, n is about 1. Propoxylated allylic alcohol can be prepared by the reaction of allylic alcohol, such as allyl alcohol and methallyl alcohol, with propylene oxide in the presence of a base catalyst. See U.S. Pat. Nos. 3,268,561 and 4,618,703.
The hydroxy-functional resin comprises recurring units of other monomers. Suitable monomers include alkyl acrylates and methacrylates, acrylic and methacrylic acids, vinyl aromatics, vinyl halides, vinyl ethers, vinyl esters, unsaturated nitrites, allyl esters, allyl ethers, allyl carbonates, conjugated dienes, and the like, and mixtures thereof.
Examples of suitable hydroxy-functional resins include homopolymers of propoxylated allylic alcohols, copolymers of propoxylated allylic alcohols and allyl esters, copolymers of styrene and propoxylated allylic alcohols, and copolymers of alkyl acrylates or methacrylates and propoxylated allylic alcohols.
Preferably, the hydroxy-functional resin has a number average molecular weight within the range of about 500 to about 10,000. More preferably, the molecular weight is within the range of about 500 to 5,000. Preferably, the hydroxy-functional resin has a hydroxyl functionality, i.e., the number of OH group per polymer chain, within the range of about 2 to about 10. More preferably, the hydroxyl functionality is within the range of about 3 to about 8. Most preferably, the hydroxyl functionality is within the range of about 3 to about 6.
Methods for making hydroxy-functional resins are known. For instance, U.S. Pat. No. 5,552,486 teaches homopolymers of propoxylated allylic alcohol. U.S. Pat. No. 5,451,652 teaches copolymers of propoxylated allylic alcohol and allylic alcohol esters. U.S. Pat. No. 5,382,642 teaches copolymers of propoxylated allylic alcohol and styrene. U.S. Pat. No. 5,475,073 teaches acrylic polyols from propoxylated allylic alcohol. The teachings of these patents are incorporated herein by reference.
Preferably, the hydroxy-functional resin is liquid at room temperature. Examples of liquid hydroxy-functional resins are homopolymers of allyl alcohol monopropoxylate, copolymers of n-butyl acrylate and allyl alcohol monopropoxylate, copolymer of 2-ethylhexyl acrylate and allyl alcohol monopropoxylate, and copolymers of allyl acetate and allyl alcohol monopropoxylate. Liquid hydroxy-functional resins produce UV-curable urethane compositions having low viscosity.
IPDI is commercially available. The molar ratio of NCO of IPDI to OH of the hydroxy-functional resin is not critical and it depends on the desired product. When the NCO/OH molar ratio is 2 or greater, almost all of the OH groups will be reacted, and thus the final urethane composition contains essentially no OH groups. If the NCO/OH is less than 2, some OH groups will not be reacted, and the final urethane contains pendant OH groups. The latter is a so-called xe2x80x9cdual-curexe2x80x9d urethane resin. It can be cured through two different mechanisms: UV-cured through the Cxe2x95x90C unsaturation, and urethane- or melamine-cured through the OH groups.
The reaction of IPDI with the hydroxy-functional resin is preferably conducted at a temperature within the range of about 25xc2x0 C. to about 125xc2x0 C. More preferably, the temperature is within the range of about 40xc2x0 C. to about 80xc2x0 C. Urethane reaction catalysts can be used to accelerate the reaction. Suitable catalysts include tertiary amines, organozinc, and organotin compounds. Examples are stannous octoate, and dibutyltin dilaurate. The amount of catalyst used is usually less than 1% of IPDI.
A solvent is preferably used in the reaction to reduce the viscosity of the reaction mixture. Suitable solvents include ethers, esters, ketones, carbonates, aromatic and aliphatic hydrocarbons, glycol ether esters, and the like, and mixtures thereof. Ketones, ethers, esters, aromatic hydrocarbons, and mixtures thereof, are preferred. The solvent can be removed after the reaction or left in the NCO prepolymer for the reaction of the NCO prepolymer with a hydroxy-functional ethylenic monomer.
Suitable hydroxy-functional ethylenic monomers include hydroxyalkyl acrylates and methacrylates, allylic alcohols, alkoxylated allylic alcohols, and the like, and mixtures thereof. Examples are hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, allyl alcohol, methallyl alcohol, propoxylated allyl alcohol, and ethoxylated allyl alcohol.
The reaction of NCO prepolymer and hydroxy-functional ethylenic monomer is preferably conducted essentially in the absence of oxygen. Preferably, a free-radical inhibitor is added to the reaction mixture to prevent the acrylate from polymerizing. Suitable free-radical inhibitors are known to the polymer industry. Examples include methyl ether hydroquinone and 2,6-di-t-butyl-4-methylphenol. Inhibitors are usually used in an amount less than 1% of the hydroxy-functional ethylenic monomer.
The reaction is preferably carried out at a temperature within the range of 25xc2x0 C. to about 125xc2x0 C. More preferably, the temperature is within the range of about 40xc2x0 C. to about 80xc2x0 C.
The method of the invention produces a fully-acrylated urethane. By xe2x80x9cfully-acrylated urethane,xe2x80x9d we mean that the urethane acrylate has essentially no OH groups. The urethane acrylate can be cured through the ethylenic unsaturation by radiation. To make a fully-acrylated urethane, the NCO/OH molar ratio in the reaction of IPDI and hydroxy-functional resin is 2 or greater.
The method of the invention also produces a dual-cure urethane. The dual-cure urethane contains both Cxe2x95x90C unsaturation, which undergoes radiation cure, and OH groups, which can be cured with isocyanates or melamines. To make a dual cure urethane acrylate, the NCO/OH molar ratio in the reaction of IPDI and hydroxy-functional resin is less than 2. Preferably, the dual cure urethane acrylate has about 3 Cxe2x95x90C and about 2 OH groups per chain.